The work in analytical biochemical laboratories and in clinical laboratories is often based on different tags or labels coupled on macromolecules under inspection. Typical labels used are different radioactive isotopes, enzymes, different fluorescent molecules and e.g. fluorescent chelates of rare earth metals. Detection of enzyme labels can be performed by utilizing its natural biochemical function, i.e. to alter the physical properties of molecules. In enzyme immunoassays colourless substances are catalysed by enzyme to colourful substances or non-fluorescent substances to fluorescent substances.
The colourful substances can be measured with absorption measurement, i.e. photometric measurement. In the absorption measurement the intensity of filtered and stabilized beam is first measured without any sample and then the sample inside one plate is measured. The absorbance i.e. the absorption values are then calculated.
The fluorescent substances can be measured with fluorescent measurement that is generally used for measuring quantities of fluorescent label substance in a sample. The most photoluminescence labels are based on molecular photoluminescence process. In this process optical radiation is absorbed by the ground state of a molecule. Due to the absorption of energy the quantum molecule rises into higher excited state. After the fast vibrational relaxation the molecule returns back to its ground state and the excess energy is released as an optical quantum. Due to losses in this process the average absorbed energies are higher than the average emitted energies.
A further measurement method is chemiluminescence measurement where emission of a substance is measured from a sample without excitation by illumination. Thus a photoluminometer suitable for photoluminescence measurements can also be used as a chemiluminometer.
Further, there is an analysing method called Amplified Luminescent Proximity Homogeneous Assay or AlphaScreen™. The function of the AlphaScreen™ method is based on the use of small beads that attach to the molecules under study. There are two types of beads that are coated with a material acting either as a donor or acceptor of singlet-state oxygen. The measurement starts, when the liquid sample is illuminated by light with a suitable wavelength e.g. 680 nm. After this, the material in the donor bead converts ambient oxygen into singlet-state oxygen. The single-state molecules have a short lifetime and they can reach only about a 200 nm distance by diffusion in the liquid. If the chemical reaction in question has taken place, both the donor and acceptor beads are bound to the same molecule and so they are sufficiently close to each other. In this case the singlet-state oxygen may reach the acceptor bead where a series of reactions is started. As the last phase of the reaction the coating material in the acceptor beads emits photons in the 500-700 nm range. If the chemical reaction has not taken place the singlet-state oxygen cannot reach the acceptor bead and the emission light is not detected. By measuring the intensity of light it is possible to conclude the efficiency of the chemical reaction.
An optical measurement instrument suitable for performing some or all of the measurements of the kind described above comprises typically at least one excitation light source for producing excitation beams to one or more samples to be measured at each time. Each excitation light source can be for example a flash lamp or a laser source. An optical path from an excitation light source to a sample may contain for example lenses, fibers, mirrors, dichroic mirrors, optical filters, monochromators and/or other optical elements. The optical measurement instrument further comprises at least one detector for detecting emission beams emitted by the samples to be measured at each time, and for producing detection signals responsive to the detected emission beams. Each detector can be for example a photodiode or a photomultiplier tube. An optical path from the sample to the detector may contain for example lenses, fibers, mirrors, dichroic mirrors, optical filters, monochromators, and/or other optical elements. The optical measurement instrument may further comprise a processing device for producing a measurement result for each sample to be measured on the basis of the detection signal related to that sample. Each sample to be measured is stored in one of a plurality of sample wells that are built on a sample well plate, e.g. a microtitration plate. The optical measurement instrument may comprise, for example, a movable sledge adapted to receive the sample well plate. Due to the fact that the sample well plate is movable, the samples can be measured in a temporally successive manner so that each sample is in turn the sample that is currently being measured and/or excited. In this document an optical module or element that is arranged to direct an excitation beam to a sample and/or to collect emission beam from a sample is called a measurement head. In other words, the measurement head is an optical interface of the optical measurement instrument with respect to a sample to be measured and/or excited.
In order to provide appropriate optical measurements, the distance from the measurement head to a sample to be measured and/or excited has to be adjusted with a sufficient accuracy. For example in conjunction with many optical measurements, an excitation beam has to be appropriately focused to the sample. The distance between the measurement head and the sample can be adjusted to a desired value for example by first moving the measurement head towards the sample well plate and, after the measurement head has touched the sample well plate, by moving the measurement head backwards away from the sample well plate with a pre-determined length of movement so as to provide the desired distance between the measurement head and the sample well plate. The situation in which the measurement head touches the sample well plate can be detected, for example, with a force sensor attached to the measurement head and arranged detect force directed by the measurement head to the sample well plate. An inconvenience related to a technical solution of the kind described above is that the measurement head can be a changeable module in which case each measurement head has to be provided with an own force sensor or, alternatively, the force sensor has to be moved from one measurement head to another measurement head every time when the measurement head is changed.